The present invention concerns photovoltaic semiconductor devices. In particular, the present invention is a method for making thin film photovoltaic devices whose absorption of incident radiation is enhanced. Examples of photovoltaic semiconductor devices include solar cells, and photodiodes. The following discussion will be directed towards a solar cell as a specific example of a photovoltaic device.
Absorption of light in a solar cell is governed by the optical properties of the semiconductor materials used to form the active layers of the cell. Optical absorption in the active layers imposes constraints on the minimum thickness of the semiconductor materials used to form the cell. Such thickness requirements impose severe purity constraints on the semiconductor materials. This follows because the purity of the active material determines the lifetime of the electron hole pair that is generated by the absorbed sunlight. The lifetime determines the average length or collection width over which carriers can be collected in the device. To maximize this requires high purity active material. Therefore, it is desirable to enhance the effective absorption within the active material because (1) the thickness of the active material can be reduced and (2) semiconductor purity requirements can be relaxed.
These considerations are particularly important for amorphous and indirect gap semiconductors. In indirect gap semiconductors, like silicon, the solar radiation is weakly absorbed. For silicon more than 50 microns of material is required to effectively absorb solar radiation. If the optical absorption were enhanced, thinner cell thickness would be required and lower purity material could be used. In amorphous silicon, blue light is absorbed in a thickness of less than 1 .mu.m while near infrared radiation (750-800 nm) requires at least 10 .mu.m for complete absorption. Since the cell must have a minimum thickness to allow for absorption of incident sunlight, the collection width must be of the order of the cell thickness to allow for the generated electron-hole pairs to contribute to the electric current. Since the collection width for amorphous silicon is at best 1.5 .mu.m, optical enhancement could provide a significant improvement in the collection efficiency of near infrared radiation. This in turn should allow the overall cell efficiency to be increased.
In the past decade, there have been a number of suggestions for the use of light trapping by total internal reflection to increase the effective absorption in the indirect gap semiconductor, crystalline silicon. The original suggestions (A. E. St. John U.S. Pat. No. 3,487,223 and O. Krumpholz and S. Moslowski, Z. Angew. Phys. 25, 156 (1968) were motivated by the prospect of increasing the response speed of silicon photodiodes while maintaining high quantum efficiency in the near infrared.
Subsequently, it was suggested (D. Redfield App. Phys. Lett. 25, 647 (1974) and D. Redfield U.S. Pat. No. 3,973,994) that light trapping would have important benefits for solar cells as well. High efficiency could be maintained while reducing the thickness of semi-conductor material required. Additionally, the constraints on the quality of the silicon could be relaxed since the diffusion length of minority carriers could be reduced proportionate to the degree of intensity enhancement. With such important advantages, interest in this approach has continued, but no significant advances in the design and fabrication of optically enhanced solar cells have been made. However, significant advances in the theoretical understanding of the light trapping problem have occurred. In particular, light trapping by total internal reflection of coherently and incoherently scattered radiation has been analyzed. Coherent scattering processes have been shown (P. Sheng, A. N. Bloch and R. Stepleman, Appl. Phys. Lett. Vol. 43, 579, 1983) to couple incident light, into guided wave modes within the semiconductor film. Coherent scattering occurs for surface textures which are periodic, such as gratings, and the well defined phase relationship of scattered waves produces constructive and destructive interference. Optimization of light trapping from coherently scattering surface textures can be difficult because scattered waves must couple to guided wave modes within the film. Coupling is accomplished by tuning the periodicity of the surface texture to match guided wave modes in a semiconductor film having a particular thickness and reflector structure. If the periodicity of the surface texture is not properly chosen, then light in the spectral region of interest will not be efficiently trapped within the film. Difficulties in tuning coherently scattering surface textures can be avoided by utilizing non-periodic surface textures which incoherently scatter light. Incoherent scattering processes tend to randomize the direction of light propagation within the semiconductor film and hence eliminate sharp constructive and destructive interferences which occur in coherent scattering processes. A statistical, mechanical analysis of the light trapping problem has shown (E. Yablonovitch and G. Cody, IEEE Trans. Electron Devices ED-29, 300 1982) that a thermodynamic limit exists for enhancement from incoherent scattering processes. Full phase base randomization of incoming light was shown to produce a maximum adsorption increase of 4n.sup.2 over that from a comparable nonscattering film, where n is the semiconductor index of refraction. This enhancement factor can be quite large, and in the case of amorphous silicon is approxiately equal to 60. Complete statistical randomization of internal light is never absolutely ensured and the 4n.sup.2 factor must be regarded as an upper limit to the absorption increase produced from incoherently scattering surface textures. Incoherently scattering surface textures are incorporated into solar cells attainable absorption increases will be significantly reduced due to optical absorption in electrical contacts and reflector structures, which is parasitic to the enhancement process. Thus, using incoherent scattering processes to enhance absorption of photovoltaic devices it is necessary to (1) produce a surface texture which efficiently randomizes the direction of light within the semiconductor layer and (2) fabricate cell geometries which minimize parasitic optical absorptions.